Rename SourceBuildUseMonoRuntime property, which is not SB specific

This is a coordinated cross-repo change, and might break the build until
all four PRs are merged. The other three are:

Ref: dotnet/source-build#4165

eng/DotNetBuild.props

@@ -53,7 +53,7 @@
       <InnerBuildArgs>$(InnerBuildArgs) $(FlagParameterPrefix)verbosity $(LogVerbosity)</InnerBuildArgs>
       <InnerBuildArgs>$(InnerBuildArgs) $(FlagParameterPrefix)nodereuse $(ArcadeFalseBoolBuildArg)</InnerBuildArgs>
       <InnerBuildArgs>$(InnerBuildArgs) $(FlagParameterPrefix)warnAsError $(ArcadeFalseBoolBuildArg)</InnerBuildArgs>
-      <InnerBuildArgs Condition="'$(SourceBuildUseMonoRuntime)' == 'true'">$(InnerBuildArgs) $(FlagParameterPrefix)usemonoruntime</InnerBuildArgs>
+      <InnerBuildArgs Condition="'$(DotNetBuildUseMonoRuntime)' == 'true'">$(InnerBuildArgs) $(FlagParameterPrefix)usemonoruntime</InnerBuildArgs>
       <!-- TODO: This parameter is only available on the Unix script. Intentional? -->
       <InnerBuildArgs Condition="'$(OS)' != 'Windows_NT'">$(InnerBuildArgs) --outputrid $(TargetRid)</InnerBuildArgs>
       <!-- PackageOS and ToolsOS control the rids of prebuilts consumed by the build.